Speed Converter

About this Speed Converter

The Speed Converter compares common road, scientific and nautical speed units. Meters per second is used as the internal reference, allowing consistent conversion among km/h, mph and knots.

Speed limits, vehicle speedometers and navigation apps display speed in different units depending on the country, with km/h standard in most of the world, mph standard in the US and UK, and knots used specifically in aviation and maritime navigation.

For related tasks, try Fuel Average Calculator, Pace Converter or Fuel Economy Converter.

How to use it

1. Enter a speed.
2. Choose the current unit.
3. Select the desired unit and convert.

Practical example

100 km/h is about 62.14 mph. One knot is one nautical mile per hour and is about 1.852 km/h.

A speed of 100 km/h converts to approximately 62.14 mph and about 53.99 knots, a conversion commonly needed by anyone renting a car abroad and needing to quickly interpret unfamiliar speed limit signs.

Things to double-check

Knots are not simply another name for mph or km/h; a knot is defined as one nautical mile per hour, and a nautical mile is longer than a standard statute mile, so knots convert to a noticeably higher km/h figure than the same numeric value in mph would.

Common speed units

One metre per second is 3.6 kilometres per hour, and one kilometre per hour is about 0.621 miles per hour. A speed of 100 km/h is about 62.14 mph, 27.78 m/s and 53.99 knots. A knot is one nautical mile per hour, about 1.852 km/h, and it is used at sea and in aviation. The converter is useful when speed limits, vehicle specifications and sport data use different units.

Driving abroad

Speed limits are signed in km/h in most countries and in mph in the United States, the United Kingdom and a few others. A 30 mph limit is about 48 km/h and a 70 mph limit is about 113 km/h. In a rental car, check whether the speedometer shows both and which scale is bigger. Fines for small excesses are real, so learn the main limits of the country in km/h or mph before you drive.

Sport and everyday speeds

A brisk walk is about 5 km/h, a recreational cyclist travels around 15 to 20 km/h and a world-class sprinter reaches over 36 km/h. The speed of sound in air is about 343 m/s, or 1,235 km/h, at 20 degrees Celsius. Putting speeds into the same unit makes comparisons meaningful, such as checking how a wind speed of 10 m/s compares with a speed limit.

Wind, weather and navigation

Wind speeds on weather forecasts may be given in km/h, m/s, mph or knots, and the Beaufort scale adds a descriptive scale. A gale is around 62 to 74 km/h. Pilots and sailors use knots because they relate to nautical miles and latitude. For passage planning, convert the speed to your usual unit and add margins for current and wind. Distances and times multiply: at 10 knots, you cover about 18.5 km in an hour.

Speed and stopping distance

Speed affects how far a vehicle travels before the driver reacts. In one second of reaction time, a car at 50 km/h covers about 13.9 metres, and at 100 km/h about 27.8 metres. Braking distance grows with the square of speed, so doubling the speed makes it about four times longer. This is why a small reduction in speed has a big effect on safety. When you read a speed limit in an unfamiliar unit, convert it so that you can judge how far you travel in a second.

Typical speeds

Walking is about 5 km/h, urban driving about 50 km/h and motorway driving about 110 to 120 km/h. A passenger jet cruises at about 900 km/h, or around 560 mph, and sound travels at about 1,235 km/h in air at room temperature. A 100 metre sprint in 9.58 seconds is an average of about 37.6 km/h. Comparing a converted figure with a speed you know well helps you check that you have chosen the right units.

Worked examples

A speed of 72 kilometres per hour is 20 metres per second, since you divide by 3.6. A car travelling at 45 miles per hour is going about 72.4 kilometres per hour. A boat moving at 30 knots is travelling at about 55.6 kilometres per hour, or 34.5 miles per hour. For quick estimates, multiply miles per hour by 1.6 to get kilometres per hour, and multiply metres per second by 3.6. The converter does the arithmetic precisely, but the shortcuts let you sense-check whether a result is sensible when you are on the road.

Journey times and what speed saves

Speed matters less than people expect over a typical trip. A journey of 450 kilometres at an average of 90 kilometres per hour takes 5 hours, and at 110 it takes about 4 hours and 5 minutes, a saving of 55 minutes. The extra speed, though, increases fuel use and risk, and stops and traffic usually reduce the average. When you plan a trip, use a realistic average speed that includes breaks, and check the legal limits along the route. Convert any limits shown in miles per hour if you are travelling abroad.

Aviation and sea speeds

Pilots and sailors use knots because one knot is one nautical mile per hour, and a nautical mile matches a minute of latitude. Aircraft have an airspeed, relative to the air, and a ground speed, relative to the earth, which differ by the wind. A plane flying at 450 knots airspeed into a 50 knot headwind has a ground speed of 400 knots. Mach numbers compare speed with the speed of sound, which varies with temperature. Conversions between knots, kilometres per hour and miles per hour are routine in navigation and in reading weather reports.

Speeds for sport and science

A sprinter who runs 100 metres in 9.58 seconds averages about 37.6 kilometres per hour, and top speed is higher still. A skydiver in a belly-to-earth position reaches a terminal speed of about 200 kilometres per hour. Light travels at about 300,000 kilometres per second, so it circles the earth about seven and a half times in a second. Sound travels at about 343 metres per second in air at room temperature. Putting speeds into a common unit lets you compare very different things on one scale.

Key terms explained

A metre per second is the standard unit of speed. A kilometre per hour is used for road limits in most countries, a mile per hour in the United States and the United Kingdom, and a knot is one nautical mile per hour. Mach one is the speed of sound, about 343 metres per second in air at 20 degrees Celsius. Ground speed is speed relative to the ground, and airspeed relative to the air. Velocity includes direction, while speed does not.

Rules of thumb and quick checks

To convert metres per second to kilometres per hour, multiply by 3.6. To convert miles per hour to kilometres per hour, multiply by 1.6. A knot is about 1.85 kilometres per hour or 1.15 miles per hour. At 100 kilometres per hour, you cover 27.8 metres each second. Use these anchors to check a result, and remember that stopping distance grows with the square of speed.

Frequently asked questions